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Tropical Highlights - July 2000

Oceanic and atmospheric conditions continued to return toward normal during July 2000, as negative sea surface temperature (SST) anomalies across the tropical central Pacific decreased in magnitude, with values more than 1.0C below normal confined to the extreme eastern equatorial Pacific (Fig. T18). Associated with this return toward normal, the Nio 3.4 and Nio 4 region indices were only 0.3C and 0.4C, respectively, during July (Table T2, Fig. T5). Across the eastern Pacific negative SST anomalies increased in magnitude, with the Nio 1+2 region index value decreasing to -1.0 (Table T2).

The oceanic thermocline remained deeper than normal in the equatorial west-central and western Pacific during the month (Fig. T15), with temperatures again averaging 3.0–4.0C above normal at thermocline depth (Fig. T17). Over the eastern and east-central Pacific, the depth of the thermocline has returned to near normal (Fig. T15), resulting in a return to near-normal temperatures in the upper 150 m of the ocean east of 140W (Fig. T17). As a result subsurface temperatures more than 1.0C below normal were only observed in the extreme eastern Pacific east of 100W. This is the first month since late 1998 that the basic east-west dipole of sub-surface temperature anomalies has been absent from the tropical Pacific.

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over the western and central equatorial Pacific during July (Fig. T25). Convection has been suppressed in this region since the beginning of the cold episode in May 1998 (Fig. T1). Over Indonesia convection was slightly below normal (Fig. T25) for the first time since September 1999 (Fig. E4). Elsewhere, convection was suppressed during the month across the monsoonal regions of India, Central America/Mexico, and the southwestern United States (Fig. T25), in contrast to the enhanced convection observed throughout these regions during June.

Consistent with the weakening pattern of negative SST anomalies across the equatorial central Pacific, the low-level (850 hPa) easterly wind anomalies have also decreased over the central Pacific since April 2000, with the zonal wind index in the region between 130E and 180 decreasing from 3.0 in April to 0.8 this month (Table T1). Across the eastern Pacific westerly anomalies were observed for the sixth consecutive month (Table T1). At upper levels, weak features replaced the well-defined upper level troughs that had persisted across the central tropical Pacific since September 1999.

The sea level pressure (SLP) pattern across the Tropics during July featured weak positive anomalies across the tropical central and eastern Pacific and negative anomalies throughout the rest of the global Tropics (Fig. T19). The Southern Oscillation Index (SOI) was negative for the second month in a row (-0.4) (Table T1), but the equatorial SOI continues to be positive (1.3), reflecting weak cold episode conditions (Fig. T2).


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